1. Field of the Invention
This invention relates in general to methods of preparing battery grade manganese dioxide and to rechargeable electrochemical cells having manganese dioxide cathodes. The invention relates more particularly to methods of preconditioning manganese dioxide to make it more suitable for use in rechargeable alkaline electrochemical cells.
2. Description of the Prior Art
Rechargeable electrochemical cells having manganese dioxide cathodes have been developed for use in a variety of applications. Such cells can be manufactured into many sizes and designs including AA, C and D size cells and complex secondary batteries capable of supplying power to large electric motors. Although cells having manganese dioxide cathodes are currently typically manufactured with zinc anodes, they can be used in association with other anodes such as cadmium anodes as well.
Because a rechargeable alkaline cell having a manganese dioxide cathode is fully charged when it is assembled, its active life starts immediately after it is produced and its first cycle in use is a discharge cycle. The net oxidation state of the manganese dioxide and hence the storage capacity of the cell are at their maximum level when the cell is first assembled.
The mechanism by which manganese dioxide discharges in an alkaline solution is somewhat complex. Unreduced commercial battery grade manganese dioxide, e.g., electrolytic manganese dioxide ("EMD"), has the formula (.gamma.-.epsilon.)-MnO.sub.x wherein x is actually typically from about 1.94 to 1.97. The net oxidation state or valence of the manganese is less than 4 (about 3.88 to 3.94). As the cell discharges, the manganese dioxide is reduced to a manganese compound having the formula MnOOH with the net valence of the manganese being about 3. If certain precautions are taken, MnOOH may be oxidized at least part way back to manganese dioxide ((.gamma.-.epsilon.)-MnO.sub.x) thus allowing the cell to be recharged. The discharge cycle of the cell must be limited or the MnOOH is further reduced to Mn(OH).sub.2, a compound having a net manganese valence of about 2. Mn(OH).sub.2 is not readily oxidized back to MnOOH or otherwise ultimately back to (.gamma.-.epsilon.)-MnO.sub.2 and therefore diminishes the capacity of the cell to be recharged. In cells such as manganese dioxide-zinc alkaline cells, the discharge cycle is typically limited by limiting the amount of zinc or other active ingredient(s) forming the anode.
A drawback to rechargeable alkaline cells having commercial battery grade manganese dioxide cathodes is that once discharged, the manganese dioxide cannot be practically recycled or recharged to its original net oxidation state (MnO.sub.1.94 to 1.97) with a conventional taper charger or otherwise using a voltage low enough for the system to handle. Using too high of a voltage causes oxygen evolution and the formation of manganate (MnO.sub.4.sup.2-) ions, both being detrimental to the system. As a result, the capacity of the cell significantly decreases as the cell is used and is not subsequently regained. In cells in which the zinc or other active ingredient(s) of the anode is limited to prevent excessive discharge of the cell, the inability of the manganese dioxide to be fully recharged prevents the zinc or other anode material(s) from being returned to its original state.
In U.S. Pat. No. 5,011,752 to Kordesch et al., it is disclosed that the above problems can be significantly reduced or eliminated by preconditioning (essentially partially reducing) the manganese dioxide forming the cathode to a compound having the formula MnO.sub.x wherein x is generally between 1.70 and 1.90, i.e., to a compound having a net manganese valence of between 3.4 and 3.8. Upon discharge of the cell, the manganese dioxide material can be recharged to an oxidation state in this range using a voltage that the system can handle. The MnOOH produced upon partially reducing the manganese dioxide prior to use of the cell acts as reserve capacity in the cathode. Thus, the storage capacity of the cell is essentially returned to its original level every time the cell is recharged thereby reducing the loss in the capacity of the cell during the life thereof. The added capacity in the discharged form allows additional recharging capacity which is important in cells manufactured with a stoichiometric deficiency of zinc or other anode material.
U.S. Pat. No. 5,011,752 states that a number of methods of preconditioning the manganese dioxide have been contemplated, including: (a) cycling the cathode in an unsealed cell, replacing the zinc anode and sealing the cell; (b) adding a reducing agent to the cathode prior to the time when the cell is finally assembled and sealed; and (c) adding an over-charge reserve material to the cathode. Reducing agents disclosed to be suitable include zinc powder, oxalic acid, ethylene glycol, hydrazine, hydrogen gas, potassium borohydride, elemental sulfur and plastic powders.
Unfortunately, it is difficult to partially reduce manganese dioxide to the desired range by some of the above methods. Some reducing agents reduce the manganese dioxide to Mn(OH).sub.2 which, as described above, cannot be effectively recharged. The use of moderate to strong acidic reducing agents or acidic environments in association therewith can cause the formation of divalent manganese (manganous) ions which are also not rechargeable and do not provide reserve capacity. Many additive components displace active ingredients of the cell thereby diminishing the capacity of the cell.
There is a need for a method of partially reducing commercial battery grade manganese dioxide (e.g., EMD) to the desired extent in a uniform and practical manner.